The present invention relates to layer-structured oxides, called an aurivillius crystal group, containing bismuth, strontium, tantalum, and oxygen, and a process of producing the oxides.
Recently, non-volatile memories made of ferroelectric thin films have been actively developed, and along with such a tendency, bismuth-strontium-tantalate, Bi.sub.2 SrTa.sub.2 O.sub.9 (hereinafter, referred to as BiSTa) having an excellent fatigue characteristic has become a focus of attention as a ferroelectric material constituting ferroelectric random access memories (FeRAMs) [see C. A-Paz de Araujo, J. D. Cuchiaro, L. D. McMillan, M. C. Scott and J. F. Scott, Nature, 374 (1995) 627; K. Amanuma, T. Hase and Y. Miyasaka, Appl. Phys. Lett., 66 (1995) 221; and S. B. Desu and D. P. Vijay, Master, Sci. and Eng., B32 (1995) 75].
With respect to BiSTa, lately, it has been reported that a thin film of BiSTa for FeRAMs was satisfactorily produced [see T. Ami, K. Hironaka, C. Isobe, N. Nagel, M. Sugiyama, Y. Ikeda, K. Watanabe, A. Machida, K. Miura and M. Tanaka, Mater. Res. Soc. Symp. Proc., 415 (1996) 195; and T. Li, Y. Zhu, S. B. Desu, C-H. Peng, M. Nagata, Appl. Phys. Lett., 68 (1996) 616].
Incidentally, BiSTa belongs to a so-called aurivillius crystal group. Various studies have been made of the aurivillius crystal group [see G. A. Smolenskii, V. A. Isupov and A. I. Agranovskaya, Soviet Phys. Solid State, 3 (1961) 651; E. C. Subbarao, Phys. Rev. 122 (1961) 804; and R. E. Newnham, R. W. Wolfe and J. F. Dorrian, Mater. Res. Bull., 6 (1971) 1029]. In addition, the aurivillius crystal group has a composition formula expressed by [Bi.sub.2 O.sub.2 ].sup.2+ [Me.sub.m-1 R.sub.m O.sub.3m+1 ].sup.2- where m is an integer of 2 or more, Me is at least one kind selected from a group consisting of sodium (Na), potassium (K), calcium (Ca), barium (Ba), strontium (Sr), lead (Pb), and bismuth (Bi), and R is at least one kind selected from a group consisting of iron (Fe), niobium (Nb), tantalum (Ta), and tungsten (W).
With respect to production of single crystals of the aurivillius crystal group, however, there have been reported only a few studies, for example, the crystallographic study of a single crystal of BiSTa by Newnham or Rae [see R. E. Newnham, R. W. Wolfe, R. S. Horsey, F. A. Diaz-Colon and M. I. Kay, Mater. Res. Bull., 8 (1973) 1183; and A. D. Rae, J. G. Thompson and R. L. Withers, Acta. Cryst., B48 (1992) 418], and the study of a single crystal of Bi.sub.4 Ti.sub.3 O.sub.12 by Dorrian or Bruton [see J. F. Dorrian, R. E. Newnham, D. K. Smith and M. I. Kay, Ferroelectrics, 3 (1971) 17; T. M. Bruton, Ferroelectrics, 7 (1974) 259].
Of the above two papers on BiSTa, the Newnham's paper did not certainly describe the composition ratio of starting materials, and the Rae's paper described the use of starting materials mixed at a constant ratio but it reported only a plate-like single crystal in a two-phase mixture state. Also, in these two papers, the analysis for characteristics of the single crystal obtained was little performed. Namely, physical properties of the aurivillius crystal group have been little known, and in recent years, there has been only barely reported a relationship between the anisotrophy and the layered structure for a single crystal of Bi.sub.4 BaTi.sub.4 O.sub.15 equivalent to the composition of the aurivillius crystal group where m=4 [see S-K. Kim, M. Miyayama and H. Yanagida, J. Ceram, Soc. Japan, 102 (1994) 722].
In these circumstances, with regard to a relationship between the composition of BiSTa and electric characteristics, it has been considered that BiSTA can exhibit a desirable remanent polarization at its stoichiometric composition [see H. Watanabe, T. Mihara, H. Yoshimori and Carios. A. Paz de Araujo, Jpn. J. Appl. Phys. 34 (1995) 5240]. Such a relationship, however, has not been fully studied, and therefore, there is a possibility that BiSTa exhibits a more desirable remanent polarization at a composition out of the stoichiometric composition.
On the other hand, the aurivillius crystal group have been considered to exhibit a ferroelectric characteristic at room temperature on the basis of the knowledge regarding the temperature dependence on a dielectric constant. However, with respect to materials belonging to the aurivillius crystal group, the number of those exhibiting ferroelectric hysteresis curves is never large, and there possibly exist those exhibiting a paraelectric characteristic. And, if there exist the materials exhibiting a paraelectric characteristic, they can extensively used for new applications different from the known ones.
The new applications may include assistants used for forming capacitors and capacitor materials for DRAMs (Dynamic Random Access Memories). In general, a capacitor is made of a ferroelectric material composed of an oxide having a perovskite structure expressed by ABO.sub.3 (for example, PZT which is a solid solution of PbTiO.sub.3 and PbZrO.sub.3, and BaTiO.sub.3) added with a suitable paraelectric material as an assistant. In most cases, a paraelectric material composed of an oxide expressed by ABO.sub.3 has been used as such an assistant, and of course, any examination has been not made to use a material belonging to the aurivillius crystal group as the assistant. However, with respect to materials belonging to the aurivillius crystal group, if there can be found those exhibiting a paraelectric characteristic, it becomes possible to produce a capacitor capable of further suppressing reduction in dielectric constant and further reducing the temperature dependency on the dielectric constant, using the material exhibiting a paraelectric characteristic as a new assistant.